U.S. patent application number 10/867251 was filed with the patent office on 2005-12-15 for fluid power accumulator using adsorption.
This patent application is currently assigned to Eaton Corporation. Invention is credited to Briggs, Roger James, Hummelt, Edward John, Kaboord, Wayne Scott, Kuznicki, Steven M., Lyman, Richard Randel JR., Talu, Orhan.
Application Number | 20050275280 10/867251 |
Document ID | / |
Family ID | 35459802 |
Filed Date | 2005-12-15 |
United States Patent
Application |
20050275280 |
Kind Code |
A1 |
Kuznicki, Steven M. ; et
al. |
December 15, 2005 |
Fluid power accumulator using adsorption
Abstract
The invention relates to a fluid power accumulator in which the
fluid undergoes a state change as the system is pressurized to
store energy. A state change can be a phase change, a chemical
reaction, or a combination of these. Generally the state change
results from the interaction of a compressible fluid contained in
the accumulator with another substance, which can be a fluid or a
solid. Preferably, the state change includes the physical
adsorption of a fluid by a solid adsorbant. The invention can
improve the energy storage density of a fluid power accumulator,
allow a given energy storage density to be achieved at a lower
maximum pressure, facilitate heat transfer and storage within an
accumulator, and/or improve accumulator efficiency by storing
energy in a form other than thermal energy, such as in the form of
chemical energy.
Inventors: |
Kuznicki, Steven M.;
(Edmonton, CA) ; Kaboord, Wayne Scott; (Mequon,
WI) ; Briggs, Roger James; (Colgate, WI) ;
Lyman, Richard Randel JR.; (Chaska, MN) ; Talu,
Orhan; (Richmond Heights, OH) ; Hummelt, Edward
John; (Greenfield, WI) |
Correspondence
Address: |
PAUL V. KELLER, LLC
4585 LIBERTY RD.
SOUTH EUCLID
OH
44121
US
|
Assignee: |
Eaton Corporation
Cleveland
OH
|
Family ID: |
35459802 |
Appl. No.: |
10/867251 |
Filed: |
June 14, 2004 |
Current U.S.
Class: |
303/11 |
Current CPC
Class: |
F15B 2201/3151 20130101;
Y02T 10/6208 20130101; F15B 1/24 20130101; F15B 2201/411 20130101;
F15B 2201/205 20130101; Y02T 10/62 20130101; B60K 6/12 20130101;
Y02E 60/16 20130101; F15B 1/04 20130101; F15B 1/165 20130101; F15B
2201/3152 20130101; F15B 2201/3153 20130101; F15B 2201/31 20130101;
Y02E 60/15 20130101; Y10S 303/11 20130101 |
Class at
Publication: |
303/011 |
International
Class: |
B60T 013/18 |
Claims
1. A fluid power accumulator, comprising: an energy storage chamber
having an enclosed volume defined in part by a mobile barrier
configured such that moving the barrier causes the enclosed volume
to increase or decrease; and a compressible fluid and a second
substance contained in the enclosed volume under pressure; wherein
the compressible fluid interacts with the second substance whereby
moving the barrier to significantly decrease the enclosed volume
causes a significant portion of the compressible fluid to undergo a
change of state and returning the mobile barrier to its previous
position substantially reverses the change of state; the change of
state comprises one or more from the group consisting of: (a) a
chemical reaction affecting the composition of the fluid; and (b) a
transition of a portion of the fluid between distinct homogeneous
portions of matter within the enclosed volume.
2. A vehicle comprising the fluid power accumulator of claim 1.
3. The fluid power accumulator of claim 1, wherein the second
substance is an adsorbant for the compressible fluid.
4. The fluid power accumulator of claim 3, wherein the adsorbant is
a molecular sieve.
5. The fluid power accumulator of claim 3, wherein the adsorbant is
carbon.
6. The fluid power accumulator of claim 3, wherein the adsorbant is
principally distributed over interior surfaces of the chamber.
7. The fluid power accumulator of claim 1, wherein: the enclosed
volume has a maximum size determined by a range of motion for the
mobile barrier; the energy storage chamber has a maximum pressure
at which it is configured to operate; and the fluid power
accumulator can store at least about 40% of a theoretical maximum
energy; wherein the theoretical maximum energy is the product of
the maximum pressure and the maximum size.
8. The fluid power accumulator of claim 1, wherein the change of
state comprises a chemical reaction.
9. The fluid power accumulator of claim 1, wherein the change of
state comprises incorporation of the compressible fluid into a
clathrate.
10. The fluid power accumulator of claim 1, wherein: the enclosed
volume comprises first and second regions; the compressible fluid
is distributed between the first and the second regions; the second
substance is contained in the second region; and the fluid power
accumulator is configured such that the barrier can be moved to
decrease the enclosed volume in such a manner that work is done on
the first and second regions and average temperatures consequently
increase within the first and second regions; wherein the work done
per unit volume on the second region exceeds the work done per unit
volume on the first region during the movement.
11. The fluid power accumulator of claim 10, wherein during the
movement the rate of temperature increase in the first region is
greater than or equal to the rate of temperature increase in the
second region.
12. A vehicle, comprising; a fluid power accumulator adapted to
store energy in a variable volume container that encloses a volume
containing matter distributed in at least two states; wherein
decreasing the enclosed volume of the container causes a
significant part of the matter in the first state to transition
into the second state; wherein the two states are either; (a) two
distinct homogeneous portions of matter within the enclosed volume;
or (b) respectively products and reactants of a reversible chemical
reaction.
13. The vehicle of claim 12, wherein the first state is a gas phase
and the second state is an adsorbed phase.
14. The vehicle of claim 13, wherein the adsorbed phase is formed
with a molecular sieve.
15. The vehicle of claim 13, wherein the adsorbed phase is formed
with high surface area carbon.
16. The vehicle of claim 13, wherein the adsorbed phase is formed
with carbon nanotubes.
17. The vehicle of claim 13, wherein the adsorbed phase is formed
with an adsorbant principally distributed near the outer surfaces
of the container.
18. The vehicle of claim 12; wherein the enclosed volume has a
maximum size; the variable volume container has a maximum pressure
at which it is configured to operate; and the fluid power
accumulator can store at least about 40% of a theoretical maximum
energy; wherein the theoretical maximum energy is the product of
the maximum pressure and the maximum size.
19. The vehicle of claim 12, wherein the transition from the first
state to the second state includes a chemical reaction.
20. The vehicle of claim 19, wherein the chemical reaction is
endothermic.
21. The vehicle of claim 12, wherein the transition from the first
state to the second state comprises the incorporation of material
into clathrate.
22. A fluid power accumulator, comprising: a housing having a first
chamber enclosing a first volume and a second chambers enclosing a
second volume, the first and second chambers being separated by a
mobile barrier, wherein moving the barrier causes the volume of the
second chamber to either decrease or; wherein the first volume
contains hydraulic fluid; the second volume contains a compressible
fluid and an adsorbant; moving the barrier to decrease the second
volume causes a significant portion of the compressible fluid to be
adsorbed by the adsorbent; and moving the barrier to increase the
second volume causes a portion of the adsorbed fluid to desorb.
23. A vehicle comprising the fluid power accumulator of claim
22.
24. The fluid power accumulator of claim 22 wherein the adsorbent
is a molecular sieve.
25. The fluid power accumulator of claim 22 wherein the adsorbent
is carbon.
26. The fluid power accumulator of claim 22 wherein the adsorbant
forms a layer over the interior of the housing.
27. The fluid power accumulator of claim 22, wherein: the second
volume has a maximum size determined by a range of motion for the
barrier: the energy storage chamber has a maximum pressure at which
it is configured operates; and the fluid power accumulator can
store at least about 40% of a theoretical maximum energy; wherein
the theoretical maximum energy is the product of the maximum
pressure and the maximum size.
28. The fluid power accumulator of claim 27, wherein the fluid
power accumulator can store at least about 50% of the theoretical
maximum energy.
29. The fluid power accumulator of claim 22, wherein the second
chamber contains a foam.
30. The fluid power accumulator of claim 22, wherein: the second
chamber comprises first and second regions; the adsorbant is
contained in the second region; and the fluid power accumulator is
operative whereby during a portion of a compression cycle, work is
done on the first and second regions and the work done per unit
volume on the second region exceeds the work done per unit volume
on the first region; wherein the compression cycle comprises moving
the mobile barrier to reduce the second volume and store energy
therein.
31. The fluid power accumulator of claim 30, wherein doing work on
the first and second regions causes average temperatures within
those regions to rise; and the system is configured such that the
rate of temperature increase in the first region during the portion
of the compression cycle is greater than or equal to the rate of
temperature increase in the second region.
32. The fluid power accumulator of claim 22, wherein: the second
chamber has a maximum volume and a minimum volume determined by a
range of motion of the mobile barrier; at maximum volume, the
compressible fluid fills at least about 50% of the volume.
33. The fluid power accumulator of claim 32, wherein at minimum
volume, at least about 30% of the compressible fluid is adsorbed.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to fluid power accumulators
and regenerative braking systems for vehicles.
BACKGROUND OF THE INVENTION
[0002] A concern relating to the use of fluid power accumulators in
smaller vehicles is the large size of the accumulators. One way to
make accumulators smaller is to operate them at higher pressure,
thereby increasing their energy storage density. There is, however,
a peak pressure at which an accumulator can operate. The peak
pressure is a design specification that must be met by plumbing,
pumps, and valves of any hydraulic systems associated with the
fluid power accumulator. Increasing the peak pressure increases the
cost of all these components.
[0003] For a given peak pressure, there is an optimal initial
charge of compressible fluid that maximizes the energy storage
capacity of the accumulator. The storage capacity is given by: 1 W
= - V 0 V 1 P V ( 1 )
[0004] Where W is the work that can be done on the system and hence
the energy that can be stored, V.sub.0 is the maximum volume for
the chamber into which the compressible fluid is charged, and
V.sub.1 is the volume of the chamber at which the maximum pressure
is reached. The pressure is a function of the volume. If the
initial fluid charge is small, the system can be extensively
compressed, but the average value of P over the volume range is
low. If the initial charge is large, the system can only be
compressed a little before the maximum pressure is reached; P is
large but the possible change in volume is small.
[0005] The variation of pressure with volume depends on the
properties of the compressible fluid, the heat capacity of the
system, and whether or not the system loses heat to the
surroundings. If the fluid behaves as an ideal gas, the
relationship between pressure and volume is given by: 2 P = nRT V (
2 )
[0006] where n is the number of moles in the gas charge, R is the
gas constant, and T is the temperature. The largest storage
capacity would be achieved if T did not increase, however, T
normally increases as the fluid is compressed. All the work done on
the system goes into thermal energy, reflected by a temperature
rise. Losing thermal energy to the surroundings is undesirable, as
the heat contains the energy stored in the system.
[0007] The temperature increase and its effect on storage capacity
can be mitigated by a foam or other agent that acts as an internal
heat sink. The benefit is offset by the volume taken up by the foam
or other agent. All things considered, a foam is generally helpful.
An optimum initial charge for the fluid power accumulator gives
about 1/3 the maximum pressure.
[0008] There continues to be a long felt need for more compact,
reliable, and efficient energy storage units for use in
regenerative braking.
SUMMARY OF THE INVENTION
[0009] The following presents a simplified summary in order to
provide a basic understanding of some aspects of the invention.
This summary is not an extensive overview of the invention. It is
intended neither to identify key or critical elements of the
invention nor to delineate the scope of the invention. Rather, the
primary purpose of this summary is to present some concepts of the
invention in a simplified form as a prelude to the more detailed
description that is presented later.
[0010] The invention relates to a fluid power accumulator in which
a portion of the compressible fluid charge undergoes a state change
as the system is pressurized to store energy. A state change can be
a phase change, a chemical reaction, or a combination of these.
Generally the state change results from the interaction of the
compressible fluid with another substance, which can be a fluid or
a solid. Preferably, the state change includes physical adsorption
of a compressible fluid by a solid adsorbant.
[0011] In one embodiment, the invention improves the energy storage
density of a fluid power accumulator. In another embodiment, the
invention allows a given energy storage density to be achieved at a
lower maximum pressure. In a further embodiment, the invention
facilitates heat transfer and storage within an accumulator having
an internal heat sink, thereby improving the efficiency of the
accumulator. In a still further embodiment, the invention improves
efficiency by storing energy in a form other than thermal energy,
such as in the form of chemical energy. One or more of these
embodiments can be combined to reduce the size, reduce the cost,
and/or improve the efficiency of fluid power accumulator-based
regenerative braking systems for vehicles.
[0012] To the accomplishment of the foregoing and related ends, the
following description and annexed drawings set forth in detail
certain illustrative aspects and implementations of the invention.
These are indicative of but a few of the various ways in which the
principles of the invention may be employed. Other aspects,
advantages and novel features of the invention will become apparent
from the following detailed description of the invention when
considered in conjunction with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a schematic illustration of a fluid power
accumulator according to one embodiment of the invention;
[0014] FIG. 2 is a schematic illustration of a fluid power
accumulator according to another embodiment of the invention;
[0015] FIG. 3 is a plot of pressure versus volume for a
conventional fluid power accumulator;
[0016] FIG. 4 is a plot of pressure versus volume for a nearly
ideal fluid power accumulator according to one aspect the present
invention;
[0017] FIG. 5 is a plot of the variation of pressure with volume as
a fluid power accumulator goes through ideal (always close to
equilibrium) and non-ideal (departing significantly from
equilibrium) operations.
[0018] FIG. 6 is a schematic illustration of a fluid power
accumulator logically divided into several regions for purposes of
discussion.
DETAILED DESCRIPTION OF THE INVENTION
[0019] One aspect of the present invention relates to a fluid power
accumulator in which a portion of a compressible fluid provided for
energy storage undergoes a change of state during compression. The
change of state reduces the tendency of the pressure to increase
with decreasing volume and thereby allows the starting pressure to
be higher for a given maximum pressure and ultimately allows the
storage density to be increased.
[0020] As the term is used here, a change of state is a change of
phase, a chemical reaction, or a combination of the two. This
definition does not include changes that affect only an intrinsic
"thermodynamic state", i.e., changes in the pressure or temperature
that do not result in net phase transitions or chemical reactions.
A phase is a distinct homogeneous portion of matter present in an
otherwise non-homogeneous physicochemical system. A phase can be,
for example, a gas phase, a liquid phase, a solid phase, or an
adsorbed phase. Examples of changes of state include the
condensation of a gas into a liquid, the adsorption of a gas onto
the surface of a solid adsorbant, the concentration of a gas inside
carbon nanotubes, and the incorporation of material into a
Clathrate. Adsorption is the preferential partitioning of a
substance from the gas phase to the surface of a solid. Adsorption
can be chemical or physical.
[0021] Most state changes that increase density are exothermic,
whereby heat associated with the state change is additive with the
heat that results from the work on the system. If the system is
improperly designed, the increase in thermal energy will overwhelm
the benefit of the density increase. A starting point for designing
an effective system is to write an expression for the change in
pressure with change in state. A system having two states can be
described in terms of three variables: the temperature, the
pressure, and x, a progress variable representing the extent of
conversion from the first phase to the second. Additional progress
variables would be added to describe a system with more than two
states.
[0022] At constant volume, for example as the system is
equilibrating following a small compression, the effect of changes
in the system variables on volume must balance, whereby: 3 ( V x )
P , T dx + ( V P ) x , T dP + ( V T ) x , P dT = 0 ( 3 )
[0023] which can be rearranged to give: 4 P x = - 1 ( V P ) x , T (
( V x ) P , T + ( V T ) x , P T x ) ( 4 )
[0024] The derivatives on the right hand side can all be determined
from the physical properties of the materials. Equations of state,
heat capacities, and enthalpies can all be estimated from readily
available data. Where the derivative of pressure with respect to
the progress variable is negative, the state change can have a
positive effect on the energy storage density.
[0025] To better illustrate the design process, a simplified
example will be considered. In this example, the first state is an
ideal gas phase and the second state is an adsorbed state or other
dense phase. The volume occupied by the adsorbed phase will be
taken as negligible, whereby the equation of state for the gas
gives: 5 V = ( 1 - x ) nRT P + V 1 ( 5 )
[0026] where x represents the fraction of the gas converted to the
adsorbed phase, n represents the initial number of moles of the
gas, and V.sub.1 is the volume occupied by the adsorbant and any
inert materials in the system. Obtaining the partial derivatives
from Equation (5), substituting them into Equation (4), and
simplifying gives: 6 P x = P ( 1 - x ) ( - 1 + ( 1 - x ) T T x ) (
6 )
[0027] The change in temperature with phase is determined by a
balance between the thermal energy released by the phase change and
the thermal energy taken up by the system:
n.DELTA.Hdx=((1-x)nC.sub.v1+xnC.sub.v2+mC.sub.v3)dT (7)
[0028] where .DELTA.H is the heat released by the phase change,
C.sub.v1 is the constant volume molar heat capacity of the gas,
C.sub.v2 is the constant volume molar heat capacity of the adsorbed
phase, m is the number of moles of an inert phase used as a heat
sink, and C.sub.v3 is the molar heat capacity of the inert phase.
Substituting this expression into equation (6) gives: 7 P x = P ( 1
- x ) ( - 1 + H TC v1 + x 1 - x TC v2 + m n ( 1 - x ) TC v3 ) ( 8
)
[0029] Where the simplifying assumption on which it is based apply,
Equation (8) can be used to design systems where a phase change
reduces the pressure increase. For example, if the first phase is
steam and the second phase is water, at 1 atm and 100.degree. C.,
.DELTA.H is about 36 kJ/mol and TC.sub.v1 is about 8 kJ/mol. If the
system is primarily steam, the first term in the denominator of the
second term in parenthesis in Equation (8) dominates the
denominator and condensation would actually increase the pressure.
On the other hand, if the system contains a lot of liquid or a lot
of a third substance that acts as an internal heat sink, then
condensation can reduce the pressure. Equations (4) and (8) can be
used to determine how much of an internal heat sink is needed to
make any given system work.
[0030] Le Chatelier's principle indicates that following a pressure
increase a state change will occur if and only if the state change
mitigates the pressure increase. In some cases, however, the state
change may occur slowly or require a spark or catalyst to initiate.
For a state change to be useful in energy storage according to the
invention, it must generally be reversible. A reversible state
change occurs quickly and spontaneously during compression and
quickly and spontaneously in the opposite direction during
expansion. If the state change does not reverse when the system is
expanded, the stored energy is generally not recovered. If the
state change occurs too slowly 5 during either compression or
expansion, the system may depart widely from equilibrium and a
significant amount of energy may be irreversibly lost through
entropy. Generally, reversible state changes include phase changes
and reactions that involve the formation or creation only of
comparatively weak bonds.
[0031] Applying Le Chatelier's principle to the steam-water system,
following a pressure increase, water will either evaporate or
condense, whichever mitigates the pressure increase. As the system
is compressed, the temperature rises and the vapor pressure of the
water increases. TC.sub.v2 for liquid water at 25.degree. C. is
about 22 kJ/mol. If the initial value for x is about 0.56 or
greater, assuming minimal loss of heat to the surroundings and
vessel walls, evaporation will mitigate the pressure increase and
water will evaporate as the system is compressed until all the
water has evaporated. Because evaporation is endothermic, the
temperature increase will also be mitigated. If the initial value
of x is less than about 0.56, condensation will mitigate the
pressure increase and steam will condense as the system is
compressed until the entire system is liquid. Regardless of the
initial value of x, this particular system is not in an appropriate
pressure range for a typical fluid power accumulator. Rather, it is
used to provide a better appreciation for the general principles of
the invention.
[0032] In a preferred embodiment, the state change induced by
compression involves a reduction in density at constant
temperature. In the evaporation example discussed above, the
reduction in pressure with state change is a consequence of heat
being taken up. The reduction in pressure can therefore be no
better than what is achieved with a large, compact internal heat
sink (assuming rapid equilibration). With a large internal heat
sink, evaporation will not occur as a result of compression.
Examples of systems in which a state change involves a reduction in
density at constant temperature include adsorption of a gas and a
chemical reaction that reduces the number of moles in a gas phase.
Where the system involves a change of phase, preferably the second
phase has at least about twice the density of the first phase, more
preferably at least about five times the density, and still more
preferably at least about ten times the density.
[0033] A further characteristic of a preferred system is one that
releases little or no heat with a density-increasing change of
state. As a rule, condensation involves a relatively large release
of heat, as does chemisorption. Physical adsorption involves a
comparatively small release of heat. Preferably, the heat released
with the state change is no more than about 30 kJ/mol, more
preferably no more than about 20 kJ/mol, still more preferably no
more than about 15 kJ/mol, and most preferably essentially no heat
is released or the state change is endothermic.
[0034] To obtain large improvements in energy storage density, it
is desirable that the effect of the state change on pressure
persist over the entire operating range. Preferably at least about
40% of the material used for energy storage undergoes the state
change over the range of operation. Where the activity of the
second state increases rapidly as material transitions to the
second state, the range where benefits are realized is reduced. For
example, where the state change is a chemical reaction and the
product is a gas, the concentration of the product gas increases
with progress of the phase change. Eventually, the product
concentration can become too great for any further energy storage
benefit to be realized. This effect is not apparent from Equation
(8), which includes simplifications that do not apply to this
example, but can be shown through a full expression of Equation
(4). Chemical adsorption is similar to a reacting system in that
the number of active sites on the surface available for the state
change decreases with progress and the number of occupied sites
available for the reverse process increases. On the other-hand, in
physical adsorption the adsorbate can form multiple layers over the
surface. Once the first layer is formed, the activity of the
adsorbed phase changes slowly with progress of the state change.
Physical adsorption systems are therefore in general better
candidates for fluid power accumulators.
[0035] It has already been mentioned that large heat releases on
state change are undesirable. A further point in this regard is
that the enthalpy of the state change is also related to the
equilibrium constant. Where there is a large heat released by a
state change, a small increase in temperature can drive the state
change in the reverse direction. Therefore, a small or negative
heat release on state change is desirable not only to avoid
releasing too much thermal energy but also to increase the
likelihood that the state change will be beneficial through the
entire range of power accumulator operation.
[0036] For all the foregoing reasons, in a preferred embodiment the
first state is a compressible fluid and the second state is an
adsorbed state of the fluid. Examples of suitable adsorbants for
forming the adsorbed state can include molecular sieves, alumina,
silica, activated carbon, and carbon nanotubes. Further options
include oxides, carbonates, and hydroxides of alkaline earth metals
such as Mg, Ca, Sr, and Be or alkali metals such as K or Ce. Still
further examples include metal phosphates, such as phosphates of
titanium and zirconium. Preferably the adsorbant has a very high
surface area. In one embodiment, the surface area of the adsorbant
is at least about 100 m.sup.2/g, in another embodiment, it is at
least about 400 m.sup.2/g, and in a further embodiment, it is at
least about 1000 m.sup.2/g.
[0037] Molecular sieves are materials having a crystalline
structure that defines internal cavities and interconnecting pores
of regular size. Zeolites are the most common example. Zeolites
have crystalline structures generally based on atoms tetrahedrally
bonded to each other with oxygen bridges. The atoms are most
commonly aluminum and silicon (giving aluminosilicates), but P, Ga,
Ge, B, Be, and other atoms can also make up the tetrahedral
framework. The properties of a zeolite may be modified by ion
exchange, for example with a rare earth metal or chromium.
[0038] For high-pressure fluid power accumulators with high energy
storage densities, a relatively inert compressible fluid is
generally preferred. Examples of relatively inert fluids include
H.sub.2, He, Ne, Ar, Xe, Kr, and N.sub.2. If a more polar fluid is
used, such as carbon dioxide, the adsorbant is preferably
relatively inert, e.g., carbon.
[0039] One example of a suitable compressible fluid-adsorbant
system effective at a peak operating pressure of around 1500 psia
is N.sub.2-zeolite. The zeolite can be Chabazite or a de-aluminated
Y-type zeolite (DAY). DAY contains a silicon to aluminum ratio of 4
or more. Another suitable fluid-adsorbant system is Ar over a very
high surface area activated carbon or carbon nanotubes. A
commercially available very high surface area activated carbon is
Norit R1, which is a carbon based on macadamia nut shells and
available through the Osaka Gas Company. The adsorbant can occupy,
for example, from about a tenth to about a third of the accumulator
maximum power accumulation chamber volume.
[0040] The adsorbant is typically combined with a binder and either
formed into a self-supporting structure or applied as a coating
over an inert substrate. A binder can be, for example, a clay, a
silicate, or a cement. Generally, the adsorbant is most effective
when a minimum of binder is used. Preferably, the adsorbant
structure contains from about 3 to about 20% binder, more
preferably from about 3 to about 12%, most preferably from about 3
to about 8%. Preferably, the adsorbant with any binder or inert
substrate is formed into a cohesive mass that resists degradation
during vehicle operation. The adsorbant structure can be fastened
directly to a vessel wall or held in place by a mesh.
[0041] The adsorbant structure preferably contains macro-pores,
whereby the entire volume of the adsorbant is accessible without
having to pass through the small pores characteristic of
high-surface area adsorbants. Such a structure can be obtained, for
example, by forming pellets of small adsorbant particles held
together with a binder. A group of pellets may themselves be formed
into a cohesive mass using more binder, or in some cases a
sintering process.
[0042] A fluid power accumulator generally includes a housing
having first and second chambers, separated by a mobile barrier.
The first chamber contains a relatively incompressible fluid (a
hydraulic fluid) and communicates with systems outside the fluid
power accumulator for energy transfer. The second chamber is
generally sealed and contains a compressible fluid. The second
chamber is where the energy is primarily stored and may be referred
to as an energy storage, chamber. Pumping hydraulic fluid into the
first chamber moves the barrier to decrease the volume of the
second chamber and compress the compressible fluid to compress.
Allowing hydraulic fluid to flow out of the first chamber moves the
barrier in the opposite direction and increases the volume of the
second chamber, causing the compressible fluid to expand. Rather
than a first chamber, a fluid power accumulator can have a
mechanical system that does not require hydraulic fluid, however,
the second chamber, the mobile barrier, and the compressible fluid
within the second chamber are always used. The mobile barrier
generally has a fixed range of motion defining a maximum volume and
a minimum volume for the energy storage chamber. A compression
cycle refers to the motion of the barrier from the point of maximum
volume to the point of minimum volume.
[0043] FIG. 1 is a schematic illustrate of an exemplary fluid power
accumulator 100 according to one aspect of the present invention.
The fluid power accumulator 100 includes housing 101 and
elastomeric mobile barrier 102. The housing 101 and mobile barrier
102 enclose a chamber 106 containing a gas 103 and an adsorbant
104. The mobile barrier 102 can be driven by hydraulic fluid 105 to
reduce the volume of the chamber 106 and compress the gas 103. As
the gas 103 compresses part of it becomes adsorbed by the adsorbant
104. The hydraulic fluid 105, which is drawn from reservoir 111, is
moved by pump 110, which can be part of a braking system. The power
stored in the accumulator 100 can be used to drive the pump 110 in
reverse, which can in turn power a vehicle.
[0044] Preferably, the adsorbant 104 is distributed around the
outer walls of the chamber 106. During compression the gas heats.
The adsorbant heats as well, but due to its thermal mass generally
heats more slowly. Over time the gas and adsorbant temperatures may
equilibrate, but having the lower temperatures near the outside
delays the loss of heat to the surroundings. Preferably the housing
101 is insulated to further reduce the loss of energy to the
surroundings.
[0045] FIG. 2 is a schematic illustration of another exemplary
fluid power accumulator 200 according to another aspect of the
invention. In FIG. 2, like elements are numbered as in FIG. 1.
Whereas the mobile barrier 102 is an elastomeric material, the
mobile barrier in FIG. 2, the piston 107, is generally rigid. A
mobile barrier can also be a metal or elastomeric diaphragm as used
in a diaphragm accumulator. Another type of mobile barrier is found
in a bellows accumulator.
[0046] FIGS. 3 and 4 together illustrate the potential improvement
in energy storage allowed by the present invention. FIG. 3 plots
pressure versus volume for a conventional fluid power accumulator
with an optimal initial charge of compressible fluid. The pressure
begins at about a third of the maximum and increases until the
fluid has compressed by a factor of three. FIG. 4 plots pressure
versus volume for a nearly ideal fluid power accumulator according
to the present invention. The pressure begins near the maximum and
remains near the maximum as the fluid is compressed through nearly
the entire volume. In both cases, the energy storage is the area
under the curve. In FIG. 4, the area is considerably greater.
[0047] The difference in energy storage potential can be more
precisely stated. Integrating Equation (1) assuming the gas is
ideal as in Equation (2) and assuming the temperature remains
constant gives: 8 W = P 0 V 0 ln ( P max P 0 ) ( 9 )
[0048] where P.sub.max is the maximum pressure, P.sub.0 is the
initial pressure, and V.sub.0 is the initial volume. While it is
conceivable that slightly more work might be obtained with an
appropriate non-ideal gas, for all practical purpose Equation (9)
represents the most energy storage that can be achieved without a
state change. Taking the derivative of Equation (9) and setting it
to zero gives the initial pressure at which the total work is
maximized, which is:
P.sub.0=0.368P.sub.max (10)
[0049] Substituting this pressure into Equation (9) gives the
maximum work for a conventional system:
W=0.368P.sub.maxV.sub.0 (11)
[0050] Generally, the actual energy storage capacity will be lower
because the compressible fluid heats on compression. With the
present invention, the work can approach:
W=P.sub.maxV.sub.0 (12)
[0051] The work in Equation (12) will be referred to as the
theoretical maximum energy storage. The theoretical maximum cannot
be achieved by the present invention, but can be approached.
Preferably, a fluid power accumulator according to the present
invention can store at least about 40% of the theoretical maximum
energy, more preferably at least about 50%, and still more
preferably at least about 60%.
[0052] While the main focus of the invention is on increased energy
storage density, another important advantage relates to rapid and
reversible storage of energy. Two factors can limit the degree to
which work done on a fluid power accumulator can be recovered as
useful energy: heat loss to the surroundings and non-equilibrium
conditions in the accumulator. Heat loss as a detrimental factor is
easily understood. The rate of heat loss can be reduced by
insulation. Another way to limit the rate of heat loss is to reduce
the system's maximum temperature by adding an internal heat sink.
An internal heat sink is a mass within the energy storage chamber.
An internal heat sink can in principle reduce the system's maximum
temperature and improve heat retention, however, an internal heat
sink may not be very effective if the system does not equilibrate
quickly.
[0053] FIG. 5 is a graph illustrating the importance of rapid
equilibration and in particular the cost of slow equilibration when
an inert heat sink is added to a fluid power accumulator. In FIG.
5, the heat sink is copper filings, which are assumed to be inert
and not functional according to the present invention. As the
system is compressed, the gas heats. If the system is compressed
very slowly, the temperatures of the gas and the copper filings may
have time to equilibrate, the temperature rise is mitigated, and
the system can essentially follow the ideal pressure versus volume
curve represented by the dashed line from point A to point B. More
realistically, the system compresses too quickly for meaningful
heat transfer to occur and the system follows the line from A to C.
Because the gas heats excessively, pressures are higher during
compression and the maximum pressure is reached at a lower fill
percent.
[0054] If after rapid compression to point C, the system is allowed
to equilibrate, the temperature of the gas drops, its pressure
drops, and the system travels from point C to point D. Point D lies
on the curve from point E to point F, which like the curve from
point A to point B is an ideal curve that can only be followed if
the system is compressed or expanded slowly enough that the system
remains essentially at equilibrium. After the system has cooled to
point D, the fluid power accumulator can receive further power to
reach point F. Point B is unattainable without loosing energy
through heat to the surroundings. If the system is decompressed
very slowly from point D or point F, the system reaches point E. At
point E, the temperature of the system is higher than the starting
temperature. The energy required to heat the system from point A to
point E equals the amount of useful energy lost during the
compression cycle due to non-equilibrium conditions. For a system
with copper filings, equilibration is likely to be too slow on both
the compression and the expansion cycles and each cycle will likely
involve irreversible losses of useful energy to heat.
[0055] The present invention provides a mechanism of facilitating
rapid thermal energy storage in a non-compressible solid with a
heat capacity that is large compared to the heat capacity of a
compressible fluid or a foam. FIG. 6 is provided to help explain
this mechanism. FIG. 6 illustrates a fluid power accumulator 60
with a piston 61 and a shell 62. The volume initially enclosed by
the shell is divided into three regions 63-65. Region 65 contains a
solid with 50% porosity. Region 65 is four time the volume of
region 64. As the piston 61 moves from position A to position B,
the gas within region 63 is forced into the regions 64 and 65. The
energy initially taken up by a region is equal to the work done at
its boundaries. The work equals the volumetric flow driven past a
boundary multiplied by the pressure at which it is driven.
[0056] A point of reference is the case where the solid is inert
and there is no state change in the gas. Consider a small movement
of the piston within region 63 where the piston begins almost at
the boundary between regions 63 and 64 (the volume of region 63 can
be neglected). The movement pushes a small volume of gas across the
boundary between regions 63 and 64. This gas distributes equally in
the free volumes of regions 64 and 65. Because the free volume of
region 64 is 8 times free volume of region 65, only one part in
nine (11%) of the gas moves across the boundary between regions 64
and regions 65. Therefore, 89% of the work done is done on region
64 and 11% is done on region 65. On a unit volume basis, twice as
much work is done on region 64.
[0057] The solid in this example is intended as an internal heat
sink. For example, the system may be designed for the internal heat
sink to take up half the thermal energy and reduce the temperature
rise by a factor of two. For the gas within region 65, heat
transfer to the solid occurs very quickly, the gas within region 65
being in intimate contact with the solid. For the gas within region
64, however, the thermal energy must migrate a considerable
distance before the solid in region 65 can take it up. According to
the design, 50% of the thermal energy goes into the solid and the
rest distributes evenly through the gas. 89% of the gas is in
region 64, therefore, taking 89% of 50%, only 45% of the energy
will be in region 64 at equilibrium. Because 89% of the energy
initially appears in region 64, 44% of the energy must migrate by
slow heat transfer processes to region 65.
[0058] According to the invention, however, some of the gas can be
taken up by an interaction between the solid and the gas such as
adsorption. If 40% of the gas displaced during compression is taken
up by adsorption and the volume of the adsorbed phase is
negligible, for a small volume of gas forced from region 63 into
region 64, 40% of the gas is taken up by the solid within region 65
and the remaining 60% distributes between the free volumes in
regions 64 and 65. One ninth of 60% is about 7%. The total flow
across the boundary between regions 64 and 65 is about 47% of the
volume driven across the boundary between regions 63 and 64.
Accordingly, 53% if the work is done on region 64 and 47% of the
work is done directly on region 65. On a unit volume basis, the
work done in region 64 is less than a third the work done on region
65. For the solid to take up 50% of the thermal energy, and the
remaining energy to be distributed evenly through the gas, only 8%
need migrate by slow heat transfer process from region 64 to region
65. A similar effect could be achieved using a reaction catalyzed
by the solid.
[0059] Thus, another aspect of the invention relates to a fluid
power accumulator comprising a energy storage chamber that can be
divided into first and second regions, the second region containing
a substance, typically an incompressible solid, not present in the
first region. During at least a portion of the compression cycle,
through an interaction between a compressible fluid and the
substance, the work done per unit volume on the second region
exceeds the work done per unit volume on the first region.
Nevertheless, due to the lower heat capacity of the first
substance, the rate of temperature rise in the first region can be
greater than or equal to that in the second region. The regions can
be defined in any suitable fashion, however, the comparison between
first and second regions cannot be made over a portion of the
compression cycle where all or part of a region falls outside the
energy storage chamber.
[0060] While most state changes that increase density are
exothermic, some state changes according to the invention can serve
to convert thermal energy into other forms. For example, consider a
reversible reaction with gas phase products and reactants: 1
[0061] Increasing the pressure will have no direct significant
effect on the activities of the products or the reactants, however,
if the reaction is significantly exothermic or endothermic there
will be an indirect effect through the temperature. As the system
temperature increases, in accord with Le Chatelier's principle, the
reaction will progress in one direction or another, whichever acts
to mitigate the temperature increase.
[0062] In a preferred embodiment, a fluid power accumulator
contains substances that react rapidly and reversibly under changes
in temperature and also contains adsorbants that are strong for one
or more products of the reaction in the endothermic direction but
weak for the products in the exothermic direction. In this manner,
a reaction can convert thermal energy into chemical energy and be
driven by both adsorption and temperature increases. A
disassociative adsorption may provide similar benefit.
[0063] The invention has been shown and described with respect to
certain aspects, examples, and embodiments. While a particular
feature of the invention may have been disclosed with respect to
only one of several aspects, examples, or embodiments, the feature
may be combined with one or more other features of the other
aspects, examples, or embodiments as may be advantageous for any
given or particular application.
* * * * *